1,3-Disubstituted imidazolium salts have been prepared by direct substitution on an imidazole unit. For example, 1-substituted imidazoles have been alkylated in the 3 position [Benac, B. L.; Burgess, E. M.; Arduengo, A. J., III Org. Syn., Vol. 64, page 92 (1986)]. This alkylation process is limited to the preparation of 1,3-disubstituted imidazolium salts for which a 1-substituted imidazole precursor is available and this imidazole precursor must be synthesized in a prior step.
Double alkylation on 1,3-disubstituted imidazoles has also been reported [Rothenberg, A. S.; Ballentine, F. A.; Panzer, H. P. Polym. Mater. Sci. Eng., Vol. 57, page 134 (1987)]. This process requires a previously formed imidazole as a starting material.
1,3-Disubstituted imidazolium salts have also been prepared by the condensation of 2 equivalents of a bis(imine) in the presence of an acid catalyst [Zettlitzer, M.; Tom Dieck, H.; Haupt, E. T. K.; Stamp, L. Chem. Ber., Vol. 119; page 1868 (1986)]. This condensation route causes the loss of one equivalent of a primary amine so that not all the available functionality can be utilized. Additionally the substituent in the 2-position of the final imidazolium salt must be derived in subsequent steps from an imine.
1,3-Disubstituted imidazole-2-thiones have been desulfurized to produce 1,3-disubstituted imidazolium salts [Wanzlick, H.; Schonherr, H. Angew. Chem., Int. Ed. Engl., Vol. 7, page 141 (1968): Wanzlick, H.; Schonherr, H. Chem. Ber., Vol. 103, page 1037 (1970): Wanzlick, H.; Schonherr, H. Justus Liebigs Ann. Chem., Vol. 731, page 176 (1970)]. This desulfurization route requires additional steps to synthesize the penultimate imidazole-2-thione and only imidazolium salts which bear a hydrogen in the 2-position can be prepared.
Imidazoles which bear no substituents in the 3-position have been synthesized from the reaction of an .alpha.-dicarbonyl compound, ammonia, an aldehyde, and a primary amine [Graf, F.; Hupfer, L.; U.S. Pat. No. 4,450,277 issued May 22, 1984]. Through this process it is not possible to synthesize imidazolium salts since there is no acid included to provide a counterion to the imidazolium group.
It is the object of this invention to provide a simple one step process for preparation of 1,3-disubstituted imidazolium salts from readily available starting materials.